Accuracy of household planning of car use: comparing prospective to actual car logs

Measures aiming at changing car use are frequently based on the assumption that car use is under motivational and volitional control. The aim of this study is to investigate the degree to which this is the case and how it relates to different purposes of car trips and characteristics of car users. Data obtained from 1-week prospective car logs made by 40 adult households living in Göteborg, Sweden, were compared to subsequent actual car use. In line with previous results suggesting that households have a modest degree of motivational and volitional control over their car use, it was found that 80% more car trips were made the following seven days than indicated in the prospective car logs. The largest discrepancies were observed for shopping and chauffeuring trips, the least discrepancy for work trips. Households having more than one car made more shopping trips than planned, households with children more chauffeuring trips than planned, and households with a higher income more leisure trips than planned. Weather conditions, illnesses, mode switches, and unspecified unexpected events were reported as reasons for the discrepancies between planned and actual trips.